1. Field of the Invention
This invention relates to an investment compound for use in a precision casting mold adapted to metals and alloys thereof such as titanium, nickel, cobalt, chromium and glass ceramics, which are relatively high in melting temperature.
2. Prior Art
Investment compounds composed, in main constituent, of quartz, cristobalite, gypsum or the like conventionally used in casting dental prostheses tend to decompose themselves or burn together with cast metal at high temperatures and particularly react heavily with active metal such as titanium at high temperatures to greatly deteriorate the cast metal. In an effort to remove such drawbacks of the investment compounds, (1) there has been developed an investment compound of an oxidation-expansion type in which magnesium oxide or zirconia is used as a main constituent and metallic zirconium is added to the main constituent for compensating the shrinkage due to solidification of the cast metal. The present inventors also proposed in Japanese Patent Application No. 213459/1986, (2) an investment compound in which magnesium oxide and aluminum oxide are used as main constituents and fine metallic titanium powder is added to aggregate so as to make it possible to reduce the amount of shrinkage of the compound and to compensate the dimensional error caused by the shrinkage due to the solidification of the cast metal.
But the above new investment compound was not free from the following drawbacks either. Namely, as proposed in the above first case, the investment compound intended to compensate for the shrinkage due to the solidification of the cast metal by the oxidation-expansion of metallic zirconium had recourses to a very cumbersome process in which, because zirconium is very expensive, a wax pattern is coated on its surface with the investment compound and then the coated wax pattern is embedded in the conventional investment compound in an attempt to make the required amount of zirconium as small as possible. Moreover, this coated layer is liable to come off when the compound is placed between the coated layer and the pattern, with the result that titanium reacts with the externally placed compound. Furthermore, when an artificial dental plate is cast, the complexity of the shape of the plate makes it fatally impossible to employ the coating method.
On the other hand, the investment compound in the above second case is less expensive than that in the first case and is greatly improved, but because the compound includes a high percentage of metallic titanium, the reaction made by the titanium with the alkaline solution produced by the reaction of magnesium oxide with water when a wax pattern is embedded generates gas which thereby produces bubbles in the mold to make the surface of castings liable to be irregular. Titanium is much cheaper than zirconium, but more expensive than ceramics for use in casting. In this sense, titanium leaves little to be desired in point of cost. Moreover, in order to provide the added titanium with sufficient expansion effect, the metallic titanium must be oxidized and must be maintained at high temperatures in that degree for a prolonged period of time, resulting in lack of convenience and economy.